MIL-STD-1553 is military standard published by the United States Department of Defense that defines the mechanical, electrical and functional characteristics of a serial data bus. It was originally designed for use with military avionics, but has also become commonly used in spacecraft on-board data handling (OBDH) subsystems, both military and civil. It features a dual redundant balanced line physical layer, a (differential) network interface, time division multiplexing, half-duplex command/response protocol and up to 31 remote terminals (devices).
A single bus consists of a wire pair with 70-85Ω impedance at 1 MHz. Transmitters and receivers couple to the bus via isolation transformers, and stub connections branch off using a pair of isolation resistors and a coupling transformer. This configuration reduces the impact of a short circuit and assures that the bus does not conduct current through the aircraft. A Manchester code is used to present both clock and data on the same wire pair and to eliminate any DC component in the signal (which cannot pass the transformers). The bit rate is 1.0 megabit per second (1 bit=1 μs).
Messages consist of one or more 16-bit words (command, data or status). Each word is preceded by a 3 μs sync pulse (1.5 μs low plus 1.5 high, which cannot occur in the Manchester code) and followed by an odd parity bit. The words within a message are transmitted contiguously and there is a 4 μs gap between messages. Devices have to start transmitting their response to a valid command within 4-12 μs and are considered to not have received a message if no response has started within 14 μs.
All communication on the bus is under the control of a master bus controller and is on the basis of a command from the master controller to a terminal (also referred to as a remote terminal (RT)) to receive or transmit. The sequence of words for transfer of data from the master controller to a terminal (in the format of sender.word-type(receiver)) is master.command(terminal)→terminal.status(master)→master.data(terminal)→master.command(terminal)→terminal.status(master). The sequence of words for terminal to terminal communication is master.command(terminal—1)→terminal—1.status(master)→master.command(terminal—2)→terminal—2.status(master)→master.command(terminal—1)→terminal_.data(terminal—2)→master.command(terminal—2)→terminal—2.status(master). The sequences ensure that the terminal is functioning and able to receive data. The status request at the end of a data transfer sequence ensures that the data has been received and that the result of the data transfer is acceptable. It is this sequence that gives MIL-STD-1553 its high integrity. The above sequences are simplified and do not show the actions to be taken in the case of an error or other fault.
A terminal device cannot originate a data transfer of itself. Requests for transmission from terminal devices are handled by the master controller polling the terminals. Higher-priority functions (for example, commands to the aircraft control surfaces) are polled more frequently. Lower-priority commands are polled less frequently. However, the standard does not specify any particular timing for any particular word, that's up to the system designers. The absence of a response when a device is polled indicates a fault.
As shown in FIG. 1, a conventional MIL-STD-1553 bus system includes a dual-redundant Mil-Std-1553 bus 14, a bus controller 10, up to thirty-one remote terminals 12 (three remote terminals 12 are represented in FIG. 1), and an optional bus monitor 16. There is only one bus controller 10 in any Mil-Std-1553-based system, and it initiates all message communication over the bus. The bus controller 10 operates according to a command list stored in its local memory, commands the various remote terminals 12 to send or receive messages, and services any requests that it receives from the remote terminals 12. The bus controller 10 also detects and recovers from errors and keeps a history of errors
A remote terminal 12 can be used to provide an interface between the Mil-Std-1553 bus 14 and an attached subsystem. For example, in a tracked vehicle, a remote terminal 12 might acquire data from an inertial navigational subsystem, and send that data over the Mil-Std-1553 bus 14 to another remote terminal 12, for display on a crew instrument. Simpler examples of remote terminals 12 might be interfaces that switch on the headlights, the landing lights, or the annunciators in an aircraft.
The bus monitor 16 cannot transmit messages over the data bus. Its primary role is to monitor and record bus transactions, without interfering with the operation of the bus controller 10 or the remote terminals 12. These recorded bus transactions can then be stored, for later off-line analysis. Ideally, a bus monitor 16 captures and records all messages sent over the Mil-Std-1553 bus 14. However recording all of the transactions on a busy data bus might be impractical, so a bus monitor 16 is often configured to record a subset of the transactions, based on some criteria provided by the application program. Alternatively, a bus monitor 16 is used in conjunction with a back-up bus controller. This allows the back-up bus controller to immediately become operatively effective if it is called upon to become the active bus controller 10.
When components are disconnected from the Mil-Std-1553 bus 14, or an equivalent bus, (resulting in an unterminated remote terminal, or open port), interference is created in the bus. For example, in the scenario where the bus 14 is used to communicate with stores on board an aircraft, such as ordinance (e.g., missiles) carried on an aircraft, and the ordinance is utilized (e.g., fired, dropped, etc.), there is a connector (previously connected to the missile) that is no longer connected to anything. As a result, signals sent down the bus 14 to this connector may propagate or reflect back along the bus 14 upon reaching the open connector, creating interference.
Conventionally, Mil-Std-1553 Data bus systems are limited to a linear topology. In other words, a non-linear topology, such as a star topology or a parallel topology, is not compatible with a 1553 bus system. As a result, the 1553 system is not compatible, for example, with the use of a carriage system of deploying weapons in which a single carriage store interface (CSI) on the bus 14 is used to communicate with multiple weapons or a single CSI on the bus is used to communicates with multiple remote terminals 12. Thus, a one-to-more than one or a one-to-many connection topology is not compatible with the conventional 1553 system or its equivalents.